(1). Field of the Invention
The present invention relates to a thickness-reduced deep-draw-formed can and a process for the preparation thereof. In the production of a thickness-reduced deep-draw-formed can by draw-redraw forming of an organic resin-coated metal plate and by reduction of the thickness by bending-elongation of a side wall portion of a can, the present invention relates to a process in which high reduction of the thickness and improvement of the corrosion resistance of the product vessel can be simultaneously attained.
(2). Description of the Related Art
As the conventional side seamless can, there can be mentioned a product formed by subjecting a metal blank such as an aluminum plate, a tinplate or a tin-free steel plate to an ironing operation in at least one stage between a drawing die and a punch to form a cup comprising a side seamless barrel and a bottom connected integrally and seamlessly to the barrel, and if desired, subjecting the barrel to an ironing operation between an ironing punch and an ironing die to reduce the thickness of the barrel of the vessel. It also is known that the thickness of the side wall can be reduced by bending-elongation by a curvature corner portion of the redrawing die instead of the above-mentioned ironing operation (see Japanese Unexamined Patent Publication No. 56-501442).
We previously proposed a process comprising holding a preliminarily drawn cup of a coated metal plate by an annular holding member inserted in the cup and a redrawing die, and relatively moving a redrawing punch, which is arranged coaxially with the holding member and redrawing die so that the redrawing punch can come into the holding member and come out therefrom, and the redrawing die so that the redrawing punch and the redrawing die are engaged with each other, to prepare a deep-draw-formed cup having a diameter smaller than the diameter of the preliminarily drawn cup, wherein the radius (R.sub.D) of curvature of a working corner portion of the redrawing die is adjusted to a value 1 to 2.9 times the thickness (t.sub.B) of the metal blank, the radius (R.sub.H) of curvature of a holding corner portion of the holding member is adjusted to a value 4.1 to 12 times the thickness (t.sub.B) of the metal blank, planar engaging portions of the holding member and redrawing die with the preliminarily drawn cup have a dynamic friction coefficient of from 0.001 to 0.2, and draw forming of at least one stage is carried out so that the redraw ratio defined by the ratio of the diameter of the shallow-draw-formed cup to the diameter of the deep-draw-formed cup is in the range of from 1.1 to 1.5, to reduce the thickness of the side wall portion of the cup by bending the side wall portion of the cup uniformly along the entire height direction.
According to this proposal, by maintaining the radius of curvature of the redrawing die and the radius of curvature of the holding member within certain ranges and maintaining the dynamic friction coefficients of the holding member and redrawing die with the preliminarily drawn cup within certain ranges, the side wall portion of the can is highly and uniformly bent to reduce the thickness of the side wall portion.
This process for the preparation of a drawn-redrawn, bent thickness-reduced can is advantageous in that a metal plate having a resin coating such as a film can be applied to the production, but since the surface pressure imposed on the preliminarily drawn cup is much larger than the surface pressure imposed in the conventional draw-redraw forming process, various troubles are caused during the forming operation.
Generally, the draw-redraw forming operation is carried out while applying a lubricant, and in view of the processability of the resin coating, it is preferred that the redraw-forming operation be carried out while heating the resin coating at a temperature higher than the glass transition temperature (Tg) of the resin. However, the conditions for reducing the thickness by the above-mentioned bending elongation are within the region of adhesion lubrication under high surface pressure and high temperature (higher than Tg) exceeding the region of complete lubrication or boundary lubrication under low surface pressure and low temperature (lower than Tg). Accordingly, the cohesion area of the resin coating to a mold or tool increases and the interfacial frictional force increases, with the result that the interfacial frictional force exceeds the shearing force among the molecular chains in the resin coating and the adhesion force of the resin coating to the metal substrate. Accordingly, breaking of the coating per se and peeling of the coating from the metal substrate are caused, and the exposed area of the metal increases and the vessel performance is degraded. Especially at a high temperature or in the case where an inclusion is present, the apparent frictional coefficient is sometimes reduced by the temperature, but this is due to reduction of the shearing force among the molecular chains and also in this case, similar problems arise from the viewpoint of the relation between the mold or tool and the resin coating.